Method and apparatus for preparing a self-curing two component powder/liquid cement

ABSTRACT

A method of preparing a self-curing two-component cement, which is particularly useful for the preparation of bone cement used in orthopedics, from a powder component and a liquid component. The method includes providing the powder component in a first at least partially evacuated inflexible container, providing the liquid component in a second inflexible container, and adding the liquid component to the evacuated powder component, such that the liquid floods the evacuated interspaces between the powder particles. An apparatus for carrying out this process is a syringe with a slidable piston, the syringe being sealed at its dispensing end by a removable plug. Preferably, the syringe is fitted with an axially collapsible spiral mixing device, one end of which is an elongated shaft which extends through the removable plug at the dispensing end. The elongated shaft can be rotated to homogenize the contents of the syringe, but during dispensing the mixing device collapses as the piston advances.

This invention relates to a method and apparatus for preparing a cementcomposed of a powder and a liquid component which polymerize whenbrought into contact with each other. The invention is especially usefulin connection with a cement which is used for medical purpose,particularly as bone cement or denture base material.

BACKGROUND OF THE INVENTION

Many modern day bone cements of the two component powder/liquid type areknown which, when thoroughly mixed together, undergo polymerizationthereby forming a hard and more or less durable cement mass.

While the present invention can be used to prepare a variety of suchcements, it is especially useful in the preparation of so-called bonecement used to anchor and support artificial joint components and otherprostheses in natural bone. Accordingly, it will be described herespecifically in that context.

The currently preferred bone cement is polymethylmethacrylate orso-called PMMA. PMMA is comprised of a powdered polymer and a liquidmonomer. Upon mixing, these components polymerize within minutes so asto form a firm rigid bond between the prosthesis and the surroundingbone structure in which the prosthesis is placed.

The present procedure for preparing PMMA bone cement is to thoroughlymix the powder and liquid component in order to start polymerizationwhereby the cement mass turns to a putty or dough consistency. Thepartially cured cement is then applied to the bone structure to betreated, e.g. into the medullary canal of the femur which is receiving afemoral shaft of a hip joint prosthesis.

All known methods for mixing bone cement have serious drawbacks, themost essential being:

poor mixing, which depends on the individual mixing technique;

high exotherm, due to the considerable amount of liquid componentnecessary to produce an applicable cement mass by conventional mixingtechniques;

creation of porosities by inclusion and entrapment of air bubbles aswell as by evaporation of excess monomer resulting in significantdegradation of the mechanical properties of the cured cement.

SUMMARY OF THE INVENTION

Accordingly, the present invention aims to provide an improved methodand apparatus for preparing a two component cement which is independentfrom the individual mixing technique.

Another object of the invention is to provide such a method which by itsreduced amount of liquid component necessary to produce an applicablecement mass lowers considerably the maximum temperature reached duringpolymerization.

Still another object of the invention is to provide such a method whicheliminates porosities in the cured cement by exclusion of air and byprevention of monomer evaporation.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a perspective view of the powder component particles in atightly packed state as used for the invention;

FIG. 2 is a sectional view of the syringe type container for the powdercomponent used in the apparatus according to the invention;

FIG. 3 shows the liquid component injection through the piston of thesyringe type container in a sectional view;

FIG. 4 shows the liquid component injection through the plug of thesyringe type container in a sectional view;

FIG. 5 is a sectional view of the syringe type container showing theinternal mixing process step;

FIG. 6 is a sectional view of the syringe type container with anadditional nozzle showing the extrusion process step;

FIG. 7 is a sectional view of the ampule used for the invention;

FIG. 8 is a diagram showing the dynamics of the flooding process of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1 the powdered component of the cement 1 ispacked as tightly as possible in the container 4 (FIG. 2) and thecontainer is evacuated. Thus void space 2 contains no, or very littleair. When allowed to enter the container 4, the liquid component willflood (arrow 3) void space 2 between the powder particles. This is theinitial phase of the mixing process producing a powder-liquid mixturewithout any, or with very little air inclusion.

FIG. 2 shows a pre-packaged powder component 1 in the container 4.Container 4 is of a syringe type with piston 5. The extrusion end 18 ofthe container is closed with a plug 6. An internal mixing device 7protrudes through the plug 6 with its shaft 8. The container isevacuated in the packaging process. For increased shelf life the outerpackage 9 is also evacuated and sealed. It may be made of aluminum foilfor example. It may also serve as the sterile outer package.

FIG. 3 shows the liquid component injection. This is done prior toapplication upon removal of the container 4 from the package 9. A needle10 is inserted through the elastomer part of piston 5. Liquid componentof the cement, which predominantly consists of methylmethacrylate 11enters the container 4 and floods as indicated by arrows 3 the powdercomponent particles 1. Flooding front 12 advances as described in moredetail later (reference is made to FIG. 8 description). Liquid component11 may be ejected as indicated by arrow 14 from its container 13 eitherby a piston if 13 is a syringe, or simply sucked out by the vacuum inthe container 4 if the container 13 is an ampule as shown in FIG. 7.

FIG. 4 shows the liquid injection through the plug 6 with the air of theneedle 10.

FIG. 5 shows additional homogenization of the mixture 15 by means of theinternal mixing device 7. Since flooding alone may not produce a mixtureuniform enough for the critical medical applications such as bonecement, means for additional mechanical mixing is provided in that anaxially collapsible mixing device 7 is enclosed and sealed within thecontainer 4. It may be made of a suitable metal, or plastic.

Its shaft 8 protrudes through the plug 6 which ensures air tight seal ofthe container 4. The shaft 8 may be gripped in the chuck of say a powerdrill and turned as indicated by arrow 16. It may also be inserted inthe specially provided mixer 17. The end of the shaft 8 may be adaptedfor simple coupling to the mixer 17. Time allowed for homogenizationdepends on the rate of the polymerization and is typically a fewminutes. Since handling of the cement is minimized and no settling timeis needed for air bubbles to come out of the mixture as it is the casein the conventional mixing, a major advantage of this procedurearises--mixing time may be increased many-fold. There is no monomerevaporation since the container 4 remains sealed throughout theinjection and homogenization phases.

FIG. 6 shows extrusion 21 of the cement following homogenization. Forthat a piston rod 20 is pushed in following removal of the plug 6.Extrusion may be done through the container end 18 or through anadditional nozzle 19 attached to the container end 18.

FIG. 7 shows a preferred embodiment of the liquid component container13. It is in the form of a glass ampule. Narrowed end 22 is shaped so asto allow needle attachment onto the conus 24. The cap 25 is broken-offat the neck 26. The needle 10 is attached to the conus 24. The ampule isthen turned upside-down and the needle 10 is inserted into the container4 through either the piston 5 or the plug 6. The cap 28 on the othernarrowed end 23 of the ampule 13 is then broken-off at the neck 27allowing the liquid 11 to flood the powder component particles 1 in thecontainer 4 as shown in FIGS. 3 and 4.

FIG. 8 shows the dynamics of the flooding process, wherein pressure dropin the injection needle 10 is neglected. Flooding front 12 position x inthe powder column 1 is proportional of the square root ot time. Theconstant of proportionality a depends on the powder column permeability,the liquid viscosity and the column cross-section. The front 12 willarrive at the end of the column x_(c) in finite time t_(c) and withfinite speed.

In the process of the present invention, preferably the weight ratio ofthe powder component to the liquid component is between 2.4 and 3.6, andmore preferably between 3.0 and 3.6. The particles of the powdercomponent are preferably approximately uniform in size and of generallyspherical form. In a preferred embodiment, the majority of powderparticles have diameters in the range of 10 to 100 μm. In a furtherpreferred embodiment, the evacuated interspaces between the powderparticles comprise 25 to 35 percent, more preferably 26 to 30 percent,of the total volume of the powder.

In another preferred embodiment, the particles of the powder componentare coated with a polymerization catalyst, preferably a peroxide. Thecement mixture of the present invention can also comprise additionalcomponents, such as radio-opaquers or antibiotics. Preferably theseadditional components are provided in a form geometrically similar tothe particles of the powder component.

I claim:
 1. A method of preparing a self-curing cement from a powdercomponent containing a polymerization catalyst and a liquid componentcontaining a polymerizable monomer, comonomer, prepolymer or mixturesthereof, said method comprising:A. supplying said powder component in anat least partially evacuated, sealed chamber of a first inflexiblecontainer, said powder component comprising particles of said powder andinterspaces therebetween; B. supplying said liquid component in a secondinflexible container; C. connecting said second container to said firstcontainer by means of an air-tight connection; D. pressurizing saidliquid component to a pressure higher than that of said partiallyevacuated chamber; and E. propelling said liquid component into saidpartially evacuated container, whereby said liquid floods said partiallyevacuated interspaces by the action of the pressure difference betweenthe pressure on the liquid component and the pressure in theinterspaces, thereby forming a polymerizable cement mass.
 2. The methodof claim 1 wherein said first container comprises a syringe, saidsyringe comprising:A. a barrel which is open at its rearward end andwhich includes a dispensing outlet at its forward end; B. piston meansreciprocably mounted in said barrel and defining a sealed chamberforward thereof for containing said powder component; C. closure meansremovably mounted on said forward end of the barrel; and D. anelastomeric injection port into said chamber, wherein said connecting,pressurizing and propelling steps comprise injecting said liquidcomponent into said chamber by means of a hollow needle inserted throughsaid elastomeric port.
 3. The method of claim 15 further comprisingmechanically mixing said polymerizable cement mass, said mechanicalmixing step comprising:providing said syringe with an axiallycollapsible, rotatable, spiral mixing member disposed in said chamberwith one end of said spiral member being elongated and extending throughsaid closure means; and rotating said elongated end of said mixingmember, thereby rotating the spiral portion of said mixing memberdisposed in said chamber.
 4. The method of claim 3 further comprisingdispensing said polymerizable cement mass, said dispensing stepcomprising:removing said closure means from said syringe while leavingsaid collapsible spiral mixing member in place; and advancing saidpiston means to force said polymerizable cement mass out of said chamberthrough said dispensing outlet.
 5. The method of claim 14 wherein saidpressurizing comprises opening said second inflexible container toatmospheric pressure, and wherein said propelling is effected by theforce of said atmospheric pressure.
 6. The method of claim 1 wherein theweight ratio of said powder component to said liquid component is in therange between 2.4 and 3.6.
 7. The method of claim 6 wherein said weightratio range is between 3.0 and 3.6.
 8. The method of claim 1 whereinsaid particles of said powder component are of approximately uniformsize and approximately spherical form.
 9. The method of claim 1 whereinthe majority of said particles of said powder component have a diameterin the range of 10 to 100 microns.
 10. The method of claim 1 wherein thevolume of said interspaces comprises 25 to 35 percent of the totalvolume of said powder component.
 11. The method of claim 10 wherein thevolume of said interspaces comprises 26 to 30 percent to the totalvolume of said powder component.
 12. The method of claim 1 wherein saidpowder particles are coated with a polymerization catalyst.
 13. Themethod of claim 12 wherein said polymerization catalyst is a peroxide.14. The method of claim 1 wherein said powder component furthercomprises radio-opaquers or antibiotics in a particulate form similar tothat of said powder particles.